1. Field of the Invention
The present invention relates to a semiconductor device and a method for manufacturing the same. Specifically, the present invention relates to a plug having a hybrid structure, in which a polycrystalline silicon plug and a metal plug are combined, and a method for manufacturing the same.
2. Description of the Related Art
JP 2008-251763 A and JP 2004-179497 A disclose a contact plug for electrically connecting a source/drain diffusion layer of an MIS transistor formed on a semiconductor substrate and a lower electrode of a capacitor. Particularly, such contact plug has a two-contact stack structure, in which a side connected to a diffusion layer of a transistor is made of polycrystalline silicon (polysilicon) and a side connected to a lower electrode of a capacitor is made of a metal conductor, such as tungsten.
Also, JP 2009-164534 A discloses a triple-layered structure comprising two contact plugs that connect a polysilicon plug and a metal plug by interposing a metal silicide layer.
Generally, a contact between metal and silicon is a Schottky contact. Therefore, as disclosed in JP 2009-164534 A, an ohmic contact successfully can be achieved by interposing a metal silicide layer between metal and silicon. In order to form a metal silicide layer on a polysilicon plug, metal, such as titanium, cobalt and nickel, for forming a silicide with silicon are formed as film on the polysilicon plug and the films are heated by lamp annealing to be silicidated, and thereafter, unreacted metal films are removed. Such a series of processes is referred to as salicidation that means self-aligned silicidation.
In a semiconductor device, particularly, in a semiconductor memory device, such as DRAM, a capacitor is formed into a three-dimensional structure to maintain a predetermined capacitance by increasing its height with miniaturizing the device. Therefore, it is necessary to form structures other than a capacitor with low height so as to prevent the increase in the thickness of structures on a substrate. In addition, a distance between contacts narrows due to refinement and the thickness of an interlayer insulating film for forming contacts also becomes thinner (the height of contacts reduces). As mentioned above, although the silicidation requires heat treatment, as the contact height lowers, an adverse effect on semiconductor elements, such as transistor, formed on a semiconductor substrate becomes large. Therefore, it is difficult to form a metal silicide layer having a sufficient thickness because the heat treatment becomes insufficient.